1. Field of the Invention
This invention relates to a shock absorber control apparatus. This invention relates to, for example, an apparatus for controlling a damping force provided by a shock absorber in a vehicle.
2. Description of the Prior Art
Japanese published unexamined patent application 3-104726 discloses a damping-force control apparatus using the sky-hook theory which is designed so that a damping force provided by a shock absorber is controlled in response to a speed of the top of a spring and a relative speed between the top and the bottom of the spring. Specifically, in the control apparatus of Japanese patent application 3-104726, the damping force is discontinuously changed between two different levels in response to the relation between the sign of the speed of the top of the spring and the sign of the relative speed between the top and the bottom of the spring. Such a control apparatus tends to be complicated since a plurality of sensors are provided to detect a speed of the top of a spring and a relative speed between the top and the bottom of the spring.
There has been a prior-art proposed method of estimating a speed of the top of a spring from a relative displacement or movement between the top and the bottom of the spring. The estimation of the speed of the top of the spring enables the omission of a related sensor from a control apparatus, allowing simplification thereof.
The prior-art proposed method will now be described further. Regarding a system including a suspension having a combination of a spring and a shock absorber designed to provide a variable damping force, an equation of motion of the system which has one degree of freedom is given as: EQU M.multidot.DDX+c.multidot.DY+k.multidot.Y=0 (1)
where "M" denotes the spring upper member weight (the weight of a member supported on the upper end or the top of the spring); DDX denotes the absolute acceleration of the spring upper member which equals the absolute acceleration of the top of the spring; "c" denotes the damping factor or coefficient of the shock absorber; DY denotes the relative speed between the spring upper member and a spring lower member (a member supported on the lower end or the bottom of the spring) which equals the relative speed between the top and the bottom of the spring; "k" denotes the spring modulus of the spring; and "Y" denotes the relative displacement between the spring upper member and the spring lower member which equals the relative displacement between the top and the bottom of the spring.
According to the sky-hook theory, an equation of motion of the system having one degree of freedom is given as: EQU M.multidot.DDX+C.multidot.DX+k.multidot.Y=0 (2)
where "C" denotes the sky-hook damping factor or coefficient, and DX denotes the absolute speed of the spring upper member which equals the absolute speed of the top of the spring. In the prior-art proposed method, the sky-hook damping factor "C" is regarded as a constant, and the term c.multidot.DY is considered to be equal to the term C.multidot.DX when DX.multidot.DY&gt;0. The equation (2) is subjected to Laplace transform, and consequently an estimated value DXp of the absolute speed of the spring upper member is given as: EQU DXp=(-k.multidot.Y/M)/(S+C/M) (3)
where "S" denotes the Laplace operator.
The following facts were experimentally found. The damping factor "c" of the shock absorber has a nonlinear relation with the relative speed DY between the spring upper member and the spring lower member. Thus, even if the sky-hook damping factor "C" is equal to a constant, the term c.multidot.DY is not always equal to the term C.multidot.DX under conditions where DX.multidot.DY&gt;0. Therefore, in the prior-art proposed method, the estimated value DXp of the absolute speed of the spring upper member tends to be low in accuracy and reliability under certain conditions.